I read that Google Books has created a searchable database of the words in its 5.2 million book digital archive, so I thought would check out “federalism” and “Federal Union” in British books and the results appear on this graph below:

“Federalism”, shown in blue, appears most in books at the end of the first world war, at the time of the Briand plan in 1930, and has its largest peak at the opening of the second world war (also the peak of activity for “Federal Union”, shown in red).

In the post-war era, peaks for federalism accompany the debates about British membership of the EEC in the 1960s and 1970s and also the end of the cold war and the agreement of the Maastricht treaty.